This invention relates to molding apparatus for producing flashless molded parts on a mold-to-mold joint surface when molding high-molecular compounds such as plastic material, rubber, etc.
Such molding apparatus for producing flashless molded parts requiring no trimming are known in U.S. Pat. Nos. 2,883,704 (hereinafter called "A"), 3,070,843 (hereinafter called "B"), 3,121,918 (hereinafter called "C"), and Japanese Unexamined Patent Publication No. 48-72258 (hereinafter called "D"), etc.
Reference "A" relates to a transfer molding apparatus, wherein a plurality of cavities are mounted on a resilient plate, in each of which a compound surpassing sufficiently a filling capacity of each cavity can be inserted and then hardened.
It is unfeasible for the above transfer molding apparatus to produce flashless molded parts effectively by reason that a certain moldable material remaining in the transfer device is forced to be hardened. Accordingly, a loss of the moldable material is large.
Reference "B" relates to improvements of the aforementioned disadvantage. After a certain amount of moldable material is filled in a cavity, only the cavity itself is transferred into a heating and thermosetting device and finally hardened therein. Also in reference "B", it is not possible to produce flashless molded parts effectively. The remaining moldable material is not hardened, but it is demanded to install at least two pressure applying devices. Accordingly, the molding cost becomes expensive.
Reference "C" is substantially identical to Japanese Examined Patent Publication No. 41-4837. According to a remarkable aspect of this technique, a chamber is mounted upon an upper surface of lower heat plate, and the upper surface of the chamber is covered by a diaphragm. A certain liquid of heat resistance is filled in the chamber and placed under a pressure application. The diaphragm is deflected by the pressurized medium or liquid in order to maintain a very close contact between the adjacent molds.
The molding technique of reference "C" is more superior to the above two references "A" and "B". And, as far as only a single or few cavities are installed to the molding apparatus according to reference "C", it is possible to produce flashless molded parts. However, this apparatus is unable to produce flashless molded parts when installing molds having many cavities.
Reference "D" relates to a molding apparatus of an elastomeric material and its process. According to the technique of reference "D", an insulating plate of heat resistance is filled between transfer means and respective curing cavity. A sufficient moldable material surpassing the capacity of the cavity is filled in the cavity. Without separating the insulating plate, only the moldable material within the cavity is hardened so as to prevent hardening of the moldable material remaining in the transfer means.
According to one aspect of this technique, a heating process and a thermosetting process are carried out continuously by one operation. However, the disadvantage is that the hardening time becomes longer and a shortage of hardening may be caused. In addition, a specifically arranged heat insulating plate must be prepared. Thus, there exists a number of inconveniences and defects.
The aforementioned known techniques are not yet actually used in the industrial world. This is likely to be due to the fact that a surface-to-surface contact of adjacent molds requires very severe accuracy, that is, an accurate parallel of not more than 1/1000 mm in a mold-to-mold joint. It may be technically feasible to produce only one flashless molded part. However, when molding a plurality of flashless molded parts simultaneously, it is required to control a surface contact of the molds with respective cavities very accurately and perfectly, but it is still technically feasible.
The defects and disadvantages of the conventional techniques may be summarized as follows:
(1) The moldable material is forced to remain in the transfer means, so that it is thermoset therewithin.
(2) In case of an injection molding, part of the moldable material remains in a sprue runner.
(3) In order to avoid the loss of the moldable material which will remain in the transfer means, various means have been developed. For example, it is necessary to install means for preventing a back flow of the moldable material, but its installation cost is expensive.
(4) In order to remove the loss of the remaining moldable material, a flexible heat-insulating material may be mounted between transfer means and a cavity. However, a heat conductivity is prevented and operational efficiency is declined. Further, since the heat-insulating material must be prepared specifically, the molding cost is increased.
(5) Any deformation of the molds as well as of the clamping means that is caused by a high heat or high pressure function at the molding time cannot be released by the remaining moldable material.
(6) In order to release the deformation, each mold is produced so as to have a certain resilience. However, it is limited to release the deformation of such a rigid mold.
(7) A chamber covered by a diaphragm is incorporated in a lower mold clamping means. According to U.S. Pat. No. 3,121,918, a heat-insulating liquid is filled in the above chamber in order to remove any deformation of the molds and of the clamping means. However, it is impossible to transmit a uniform pressure force to the cavity by the following reasons:
(i) The diaphragm must be of considerable strength in order to cope with a mold clamping force. Accordingly, it must be made of a metal material. Needless to say, the metal-made diaphragm is not able to have a sufficient flexibility. When using such a diaphragm, it is impossible to carry out a slight control of a 1/1000 mm level in a surface-to-surface joint of a pair of molds.
(ii) As far as any one of the molds having a larger dimensional error or a parallel degree of the mold surface being in disorder, it is impossible to obtain a complete surface-to-surface joint of adjacent molds.
(iii) Since such a metal-made diaphragm has no sufficient flexibility, it is impossible to mount a large number of molds upon the diaphragm. Only a few molds may be mounted thereupon.